IVF treatment Hormone stimulation, ultrasound checks, collection of oocytes, and fertilization will follow standard procedure. For women in the control group, all expanded blastocysts will be cryopreserved (vitrified) day 5-6 for later transfer. For women randomized to PGS, cells (trophectoderm cells in the outer cell layer of the embryo) will be biopsied on day 5-6 after which all biopsied blastocysts will be vitrified. If the analysis shows a normal chromosome set, the blastocyst will be thawed and transferred during the next menstrual cycle.